Solvent-free hydrosilylation of alkenes and alkynes using recyclable platinum on carbon nanotubes

Article abstract of DOI:10.1039/d0gc03943h

Platinum nanoparticles were stabilized at the surface of carbon nanotubes and the nanohybrid was valorized as a catalyst for the hydrosilylation of alkenes and alkynes. The heterogeneous catalyst operated under sustainable conditions (room temperature, no solvent, low catalyst loading, air atmosphere) and exhibited improved stabilty as recycling and reuse could be achieved for multiple consecutive reactions.

Full text of DOI:10.1039/d0gc03943h

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Solvent-free hydrosilylation of alkenes and alkynes
using recyclable platinum on carbon nanotubes†
Cite this: Green Chem., 2021, 23, 815
Dhanaji V. Jawale, a Valérie Geertsen, b Frédéric Miserque,c
a
Patrick Berthault, b Edmond Gravel *a and Eric Doris
*
Received 20th November 2020,
Accepted 12th January 2021
Platinum nanoparticles were stabilized at the surface of carbon nanotubes and the nanohybrid was valor-
ized as a catalyst for the hydrosilylation of alkenes and alkynes. The heterogeneous catalyst operated
under sustainable conditions (room temperature, no solvent, low catalyst loading, air atmosphere) and
exhibited improved stabilty as recycling and reuse could be achieved for multiple consecutive reactions.
DOI: 10.1039/d0gc03943h
rsc.li/greenchem
lysts,12 including Pt-based,13 have been reported for the hydro-
silylation of olefins, but the latter species classically require
Introduction
The hydrosilylation of olefins1 is a key transformation in specific and sometime constraining reaction conditions. Thus,
today’s chemical industry, applied to the production of orga- the development of efficient heterogeneous hydrosilylation cat-
nosilicon-based materials such as rubbers, adhesives, coat- alysts, operating under mild conditions, and which can be
ings, cosmetics, and healthcare-related products.2 The reaction readily recycled is still sought.
classically consists in the addition of hydrosilanes (R3Si–H) to
Our group has long been active in the development of semi-
alkenes or alkynes, using radical-3,4 or transition metal- heterogeneous14 and heterogeneous catalysts.15 For example,
mediated processes.5 While the former route is of limited different noble16 and base17 metals have been assembled on
scope, metal catalysis has found broader uses, thanks to carbon nanotubes and studied as heterogeneous catalysts for
homogeneous complexes.6 In that respect, noble metals various organic transformations. In the present work, we
(mainly Pt and Pd) were shown to be the most efficient, report a new catalytic platform obtained from the immobiliz-
especially in terms of selectivity. Homogeneous platinum cata- ation of platinum nanoparticles (PtNPs) on carbon nanotubes
lysts such as those developed by Speier7 or Karstedt8 are nowa- (CNTs). The PtCNT nanohybrid was investigated for the cataly-
days widely employed in the industry9 and, despite potential sis of the hydrosilylation reaction of alkenes and alkynes, with
occurrence of side reactions, these complexes usually exhibit emphasis on performances, sustainability and recyclability.
satisfactory performances under mild conditions. Yet, homo-
geneous metal complexes suffer from lack of sustainability as
they cannot be recovered and/or recycled. As a result, the Si
Results and discussion
industry wastes over five tons of platinum every year,10 and the
cost of the catalysts accounts for a significant fraction of the
market price of silicones.11 Another important issue is the
potential contamination of the products by the soluble metal
catalysts which could end-up trapped in the silicon material.
As opposed to homogeneous catalytic complexes, hetero-
geneous catalysts are more durable as they can be recovered
from reaction mixtures and reused. Several supported cata-
The PtCNT nanohybrid catalyst was assembled according to
our previously reported procedure that was adapted for
PtNPs.16a In short, multiwalled carbon nanotubes were dis-
persed in an aqueous suspension of amphiphilic molecules
(DANTA) bearing a polar nitrilotriacetic head group and a dia-
cetylene lipophilic chain. Upon sonication, DANTA self-orga-
nized at the surface of CNTs, as hemi-micellar ring-like struc-
tures, with their lipophilic chains in contact with the hydro-
phobic surface of CNTs and polar heads interfacing with
water. The rings were subsequently polymerized by UV
irradiation of the diacetylene units, which reinforced the
assembly around the nanotube. Coating with DANTA resulted
in a high density of anionic carboxylates at the surface of the
CNTs that allowed ensuing deposition, through electrostatic
interactions, of a second layer of cationic polydimethyl-
diallylammonium chloride polymer (PDADMAC). Finally, pre-
aUniversité Paris-Saclay, CEA, INRAE, Département Médicaments et Technologies
pour la Santé (DMTS), SCBM, 91191 Gif-sur-Yvette, France.
E-mail: edmond.gravel@cea.fr, eric.doris@cea.fr
bUniversité Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France
cUniversité Paris-Saclay, CEA, DES-Service de la Corrosion et du Comportement des
Matériaux dans leur Environnement (SCCME), 91191 Gif-sur-Yvette, France
†Electronic supplementary information (ESI) available: Analytical data and
copies of NMR spectra. See DOI: 10.1039/d0gc03943h
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formed platinum nanoparticles18 were added to the aqueous X-ray photoelectron spectroscopy (XPS) confirmed the presence
multi-layer system. The tridimensional polyammonium of Pt(0) as the main species, together with some higher oxi-
network acts here as an anchoring and stabilization unit for dation state derivatives (i.e. Pt(II) and Pt(IV), see Fig. S1†). In
platinum nanoparticles (Fig. 1).
parallel, platinum content of the aqueous suspension was
The resulting hybrid material was characterized by trans- measured by inductively coupled plasma mass spectrometry
mission electron microscopy (TEM, Fig. 2) which showed (ICP-MS, [Pt] = 4 mM).
dense and homogeneous covering (Fig. 2b) of the CNT surface
With the PtCNT catalyst in hands, we assessed its efficacy
with Pt nanoparticles of 2.1 nm average diameter (Fig. 2c). in the hydrosilylation reaction of 1-hexene (1a) by triethoxysi-
lane (2a), used as model substrates. The reaction was set by
working at room temperature, under atmospheric pressure of
air, using 0.04 mol% of PtCNT and without any solvent (water
from the catalyst was removed by acetone washings). Under
these reaction conditions, hydrosilylation proceeded smoothly
to give the expected hydrosilylated product 3aa in 98% yield
after 24 h (Table 1, entry 1). Control experiments were carried
out using either (i) free standing colloidal Pt nanoparticles; (ii)
multi-layer decorated CNTs incorporating no platinum; (iii)
platinum salts precursor to the PtNPs (PtCl4); or (iv) platinum
on charcoal (Pt/C). However, none of these control experiments
(entries 2–5) led to any conversion of 1a into 3aa, thus high-
lighting the effectiveness of PtCNT under mild conditions.
To establish the heterogeneous nature of the PtCNT cata-
lyst, a classical reaction was set under the conditions of entry
1, Table 1 (1a/2a/PtCNT, no solvent, r.t.) and run for 3 h. At
this point the mixture was filtered on a 0.45 µm membrane to
remove the heterogeneous PtCNT. A fraction of the filtrate was
1
analyzed by H NMR which indicated approximately 15% con-
version of the substrate. The remaining filtrate was further
stirred for another 24 h, but no additional conversion into 3aa
was detected. This experiment suggests that hydrosilylation is
catalyzed at the surface of the nanohybrid and not by an active
species that could have leached from PtCNT. In addition, no Pt
Fig. 1 (a) Schematic representation of the PtCNT hybrid; (b) chemical
structure of DANTA; (c) chemical structure of PDADMAC.
was detected by ICP-MS analysis of the filtered reaction
mixture.
We next turned our attention to the scope and limitations
of the process and first investigated the reaction of 1-hexene
1a with different silane partners (Table 2, entries 1–5). Under
the above conditions, phenyldimethylsilane 2b (entry 2),
benzyldimethylsilane 2c (entry 3), and chlorodimethylsilane 2d
(entry 4) afforded the expected product 3ad in high yield (94–97%)
Table 1 Different conditions investigated for the hydrosilylation
reactiona
Entry
Catalyst
Loading (mol%)
Yieldb (%)
1
2
3
4
5
PtCNT
0.04
0.04
—
0.04
0.04
98
0
0
0
0
Colloidal PtNPs
Coated CNT
PtCl4
Pt/C
Fig. 2 (a) TEM image of the PtCNT hybrid; (b) magnification of the
boxed area; (c) size distribution of Pt nanoparticles (based on the
measurement of 250 particles from TEM micrographs).
a
1a (1 mmol), 2a (1.2 mmol), catalyst, room temperature, 24 h. b Yield
of isolated product.
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Table 2 Scope of the PtCNT-catalyzed hydrosilylation of aliphatic substratesa
Entry
Substrate
1
Silane
2
Product 3
t (h)
Yieldb (%)
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1b
1b
1c
HSi(OEt)3
2a
2b
2c
2d
2e
2d
2b
2b
3aa
3ab
3ac
3ad
3ae
3bd
3bb
3cb
24
24
24
24
30
24
24
24
98
94
97
97
91
99
98
92
HSi(Me)2Ph
HSi(Me)2Bn
HSi(Me)2Cl
HSi(Et)3
HSi(Me)2Cl
HSi(Me)2Ph
HSi(Me)2Ph
9
1d
1e
HSi(Me)2Ph
HSi(Me)2Ph
2b
2b
3db
3eb
24
24
93
98
10
11
12
1f
HSi(Me)2Ph
HSi(OEt)3
2b
2a
3fb
3ga
24
30
98
87
1g
13
14
1h
1i
HSi(Me)2Ph
HSi(Me)2Ph
2b
2b
3hb
3ib
48
24
—
97
15
16
1j
HSi(Me)2Cl
HSi(Me)2Ph
2d
2b
3jd
24
36
96c
93c
1k
3kb
a
1 (1 mmol), 2 (1.2 mmol), neat, PtCNT (0.04 mol%), room temperature. b Yield of isolated product. c E-Isomer is formed exclusively.
and in 24 h. In the case of triethylsilane 2e (entry 5), the reac- The latter reactions with alkynes led selectively to the
tion was successful but required 30 h to afford 3ae in compar- E-substituted alkenes.
able yield (91%). Having validated the system with various
The PtCNT catalyst was in parallel also investigated for the
silanes, we next looked at different olefinic partners, starting hydrosilylation of aromatic substrates (Table 3). The process
with aliphatic terminal alkenes (entries 6–12). 1-Octadecene 1b was found to be operative on a variety of styrene derivatives
(entries 6 and 7) underwent reaction with silane 2d or 2b in such as plain styrene 4a (entry 1), p-chlorostyrene 4b (entry 2),
excellent 99 and 98% yield, respectively. Other alkenes bearing p-methoxystyrene 4c (entry 3), p-vinylphenylboronic acid 4d
sensitive functional groups were also investigated, such as (entry 4), and vinylnaphtalene 4e (entry 5) which all readily
6-heptenoic acid 1c (entry 8), 4-penten-1-ol 1d (entry 9), underwent reaction with silane 2b in excellent 96–98% yield.
5-hexen-2-one 1e (entry 10), and 1,2-epoxy-5-hexene 1f (entry 11). Phenylacetylene 4f was also smoothly converted to the corres-
The latter substrates readily reacted with silane 2b to yield the ponding hydrosilylated product 5fd as a single E-isomer,
corresponding hydrosilylated products in 92–98% yield. Of after 30 h of reaction (93%, entry 6). In the aforementioned
note, no alteration of the functional groups (carboxyl, alcohol, examples, the hydrosilylation reaction is taking place in an
epoxide, ketone) was observed. The same comments hold true anti-Markovnikov fashion, leading exclusively to the
for diethyl allylmalonate 1g whose reaction with triethoxysi- C-terminal-silylated products. Finally, the reactivity of the
lane 2a afforded 3ga in 87% yield after 30 h of reaction internal alkyne 1,2-diphenylethyne was scrutinized (entry 7).
(entry 12).
Upon reaction with phenyldimethylsilane, hydrosilylated 1,2-
The scope was extended to cyclic alkenes but the reaction of diphenylethene 5gb was produced in 97% yield as the
cyclohexene 1h with phenyldimethylsilane 2b failed (entry 13), E-isomer.
while that of norbornene 1i led to the clean formation of 3ib
To further illustrate the applicability of the PtCNT-mediated
in nearly quantitative yield, after 24 h (entry 14). The above process, we successfully performed the hydrosilylation of more
process is not only restricted to alkenes but could also be challenging substrates (Fig. 3). For instance, para-bis(dimethyl-
applied to alkynes such as 1-hexyne 1j (reacted with silane 2d, silyl)benzene was converted to double-hydrosilylated 3af in
96% yield of 3jd, entry 15) or 1-ethynyl-1-cyclohexanol 1k 97% yield by reaction with 2 equivalents of 1-hexene 1a.
which was readily converted to 3kb in 93% yield (entry 16). Similarly, antimalarial alkaloid quinine underwent smooth
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Table 3 Scope of the PtCNT-catalyzed hydrosilylation of styrene derivativesa
Entry
1
Substrate
4
Silane
2
Product 5
t (h)
Yieldb (%)
97
4a
HSi(Me)2Ph
2b
5ab
24
2
3
4
5
4b
4c
4d
4e
HSi(Me)2Ph
HSi(Me)2Ph
HSi(Me)2Ph
HSi(Me)2Ph
2b
2b
2b
2b
5bb
5cb
5db
5eb
24
24
24
24
96
97
98
96
6
7
4f
HSi(Me)2Cl
HSi(Me)2Ph
2d
2b
5fd
30
24
93c
97d
4g
5gb
a
4 (1 mmol), 2 (1.2 mmol), neat, PtCNT (0.04 mol%), room temperature. b Yield of isolated product. c E-Isomer is formed exclusively. d E-Isomer
formed exclusively.
Fig. 3 Examples of PtCNT-based hydrosilylation on structurally chal-
lenging substrates.
Fig. 4 Postulated mechanism for the PtCNT-mediated reaction, based
on deuteration experiments and previous reports.
hydrosilylation to afford compound 3lb in excellent 95% yield.
Monoterpene L-carvone, bearing a terminal double bond and
carried out to afford, after 24 h of reaction, the deuterated
product 3ab′ in 24% yield (vs. 94% for phenyldimethylsilane
2b). A marked difference in reaction rates was observed and a
kinetic isotope effect (KIE) was calculated based on the deuter-
iosilane/hydrosilane rate constant ratio (kH/kD = 3.7, Fig. S3†).
This value suggests that Si–H(D) bond dissociation is the rate-
determining step. The exclusive formation of 3ab′ is in agree-
ment with the accepted mechanism of hydrosilylation (here
deuterosilylation, Fig. 4) and involves: (i) oxidative addition of
2a′ (step ①), (ii) coordination of olefin 1a (step ②), (iii)
migratory insertion of the alkene in the Pt–D bond (deutero-
metallation, step ③), and (iv) reductive elimination to afford
an enone, was selectively transformed into compound 3mb (as
a mixture of two diastereomers) in 92% yield. So was the alkyne-
bearing progestogen ethisterone whose hydrosilylated product
5hb was obtained as a unique E-isomer in 87% yield. The
above examples show the versatility of PtCNT in promoting
hydrosilylations of a wide variety of molecules.
The mechanism of homogeneous Pt-based hydrosilylation
has been previously investigated and discussed by several
groups, which resulted in a revised version of the Chalk–
Harrod mechanism.19 To find out whether our newly devel-
oped process follows a similar path, an experiment involving
deuterated phenyldimethylsilane 2b′ and 1-hexene 1a was
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the final product 3ab′ and regenerate the active catalyst (step
④). While steps ① and ② are most likely fully reversible,
olefin insertion in step ③ is moderately prone to reversibility,
as opposed to what was initially proposed for the Chalk–
Experimental
General procedure for hydrosilylation reactions (the reaction
of 1a with 2b is given as a representative example)
Harrod mechanism.20 In fact, only trace amounts (<5%) of 100 µL of colloidal PtCNT in water are transferred to the reac-
deuterated olefin 1a′ were detected as a result of the partial tion vessel before 1 mL of acetone is added. The supernatant
reversibility of step ③.
is discarded and the washing process is repeated twice. The
In addition to environment-friendly conditions, the catalyst is finally dried under a flow of nitrogen gas. Under air,
efficient recycling of the precious metal-based catalysts is of to the water-free catalyst (0.04 mol%) was added 1-hexene 1a
utmost importance for the development of new sustainable (84 mg, 1 mmol) and phenyldimethylsilane 2b (163 mg,
chemical processes. To demonstrate the recyclability of the 1.2 mmol). The reaction mixture was stirred for 24 h at room
PtCNT nanohybrid, six consecutive hydrosilylation reactions temperature. CH2Cl2 (1 mL) was added and the suspension
of 1-hexene 1a with chlorodimethylsilane 2d were carried out was centrifuged (2000g, 5 min). The supernatant was collected
using the same batch of catalyst. At the end of each run, the and the precipitated catalyst was washed with CH2Cl2 (2 ×
hybrid catalyst was recovered by simple benchtop centrifu- 1 mL). The collected organic layers were combined, dried on
gation and reused in the next reaction. Gratifyingly, the cata- MgSO4, filtered, and concentrated under reduced pressure to
lytic performances of the PtCNT hybrid remained consistent give 3ab as a colorless viscous liquid (94%).
over the course of the experiments, affording the desired 3ad
product in yields comprised between 96 and 98% (Table S1†).
Recycling experiments
TEM images of the used catalyst showed no alteration of the Under air, 1-hexene 1a (84 mg, 1 mmol) and chlorodimethyl-
nanohybrid morphology and Pt(0) was again detected as the silane 2d (113 mg, 1.2 mmol) are added to water-free PtCNT
main species by XPS (Fig. S2†). Of note, the recycling of the (0.04 mol%). The reaction mixture was stirred for 24 h at room
PtCNT catalyst neither required complicated purification nor temperature. CH2Cl2 (1 mL) was added and the suspension
reactivation steps. In fact, simple extraction of the product was centrifuged (2000g, 5 min). The supernatant was collected
with an organic solvent was the only workup needed to and the precipitated catalyst was washed with CH2Cl2 (2 ×
proceed with subsequent experiments.
1 mL). The collected organic layers were combined, dried on
Although different metal-based heterogeneous catalysts MgSO4, filtered, and concentrated under reduced pressure to
have been reported in the literature for the hydrosilylation of give 3ad. The pelleted catalyst was simply reused in sub-
olefins, the nanohybrid developed in this work compares sequent reactions by dispersion in fresh reagents (1a and 2d).
favorably with previously known systems. In fact, some other
metal-based catalysts operate under harsher conditions of
temperature21 and pressure,22 require an inert atmosphere,23 Conclusions
or fall short in terms of stability24 and/or recyclability.21b,25 In
A heterogeneous platinum-based catalyst was assembled from
addition, in some cases, higher catalytic loadings and sol-
a carbon nanotube platform. The resulting nanohybrid was
vents23 are required. Some of the above comments also apply
evaluated for the solvent-free hydrosilylation of alkenes and
to closely related carbon-supported catalysts, i.e. platinum on
alkynes under mild conditions (room temperature, 0.04 mol%
charcoal or platinum on graphene,26 as the latter operate
catalyst loading, air atmosphere). The process is not only of
under more drastic conditions and require higher catalytic
wide scope and high efficiency (including on more complex
loadings. Taken together, these shortcomings highlight the
effectiveness of the PtCNT catalyst which is active at room
molecules bearing sensitive groups) but also sustainable as re-
cycling and reuse of the nanohybrid was readily achieved with
temperature and under air, requires as little as 0.04 mol%
no alteration of the catalytic performances. In that sense, the
Pt-loading, is active in the absence of solvent, and can be
results obtained in the context of this study compare favorably
readily recycled and reused with no loss of catalytic per-
formances. In addition, the nanohybrid-catalyzed hydro-
to previous reports.
silylation reaction can be run on a larger scale (e.g.
10 mmol of 1a + 2b), similarly providing access to 3ab in
excellent yield (96%) and catalytic loadings can be
Conflicts of interest
decreased down to 0.004 mol%, although 60 h are needed
here to achieve full conversion. The superior activity
of PtCNT could be rationalized by the fact that carbon
There are no conflicts to declare.
nanotubes are electronically active and are likely to stabilize
higher transient oxidation states of platinum by collabora-
Acknowledgements
tive interactions with the metal.27 In addition, stabilization D. V. J. thanks the “Make our Planet Great Again” program and
of Pt nanoparticles in the tridimensional polyammonium the “Initiative de Recherche Stratégique MOMENTOM” for
coating of the nanotube prevents their agglomeration and support. The “Service de Chimie Bioorganique et de
deactivation.
Marquage” (SCBM) belongs to the Laboratory of Excellence in
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Research on Medication and Innovative Therapeutics (ANR-10-
LABX-0033-LERMIT) and is a partner of NOMATEN, a Centre
of Excellence in Multifunctional Materials for Industrial and
Medical Applications. The authors would like to acknowledge
the help of the TEM Team of the Joliot Institute for the acqui-
sition of transmission electron microscopy images. Mrs Amélie
Commun., 2014, 50, 15251; (c) D. V. Jawale, E. Gravel,
C.
Boudet,
N.
Shah,
V.
Geertsen,
H.
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I. N. N. Namboothiri, E. Gravel and E. Doris,
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